Using primary as the secondary winding in the power transformer.

[mishak]

I know it is not the way how it should be used, but I wonder what are the negative arguments against using primary as the secondary winding in the power transformer. My situation is: I want to build a power supply for the Technics SP-10 MKII turntable, and it needs 10VAC, 50VAC, and 110VAC all total less than 50W.

I see on ebay good quality transformers with 10VAC, 50VAC secondaries, and two primary 110VAC. So I thought I could use that separate 110 VAC coil as a secondary.
It suppose to feed neon lamp only with current less than 0.1 A.

Can I do it?

[pilli]

At a first level of analysis:
The windings don't "know" what they doing, whether they are inducing a magnetic field because of a variable current running in them, or whether a current is being induced in them because they're inside a variable magnetic field. As long as you respect "maximums", it will work. (But make sure you understand to what these apply, eg. on 110V input rather than 220V...)

Then, to go more into detail (but others, more expert, may describe it better than I do) I think different types of transformers are physically built with different purposes in mind, to go for specific properties. For example, power supply transformers may have the bobbins meant to be "primaries" and "secondaries" physically separated (for further isolation?), or signal transformers may have bobbins "layered" for specific signal transfer characteristics.

I think that if you stay very conservative with the "max values" (as is the case in your example) then it should be ok.


_

[davidsrsb]

There may not be that much safety isolation - probably just 1.5KV, between the two 110V windings, but otherwise everything should work

[mishak]

So safety will be the main issue here, and there is no way to know it unless manufacturer would provide that data for a given winding of transformer, correct?
Here is the one I was looking for:
Toroid Transformer T20 65W for TDA1541 kit - eBay (item 280395724172 end time Jun-07-10 08:04:13 PDT)

[woody]

You also might look under Miscellaneous at Apexjr.com he has a lot of transformers
and you might find a cheaper way to do this if you use a pair of transformers.

[mishak]

Thank you, woody.
I've checked, they don't have suitable for me combination

[Speedskater]

I don't think that it would be a problem is this circuit, but don't modern power transformers have a fused primary?

[davidsrsb]

Transformers do have a single thermal fuse in the primary winding.
This does not provide any safety isolation from at least one of the live side wires

If your project does not have safety earth, use a proper isolating transformer for peace of mind

[Zero Cool]

This works MOST of the time. there are some issues however. you can only push this so far. for example. if you have a 12V secondary and tried to run that on 120V so that the primary of 120 became much much higher in voltage. There is a point where you reach core saturation and you start drawing lots of line current and things blow up. the ratio of primary to secondary and other factors come into play.

running a 120/240 transformer either way works but running a 120V transformer on 240V may not.

Also EI VS toroid makes a difference. I had some beautiful 2Kva dual 120 dual 240V transformers i wanted to run backwards to have dual 60V and was advised against it. something about the core not being fully energized etc etc...

If you are only running a neon lamp off that extra primary, it shouldn't be an issue at all.

Or if you want i have some new small 115v/115v 15 watt iso transformers i could sell you for a few bucks.

[abraxalito]

Originally Posted by mishak

I see on ebay good quality transformers with 10VAC, 50VAC secondaries, and two primary 110VAC. So I thought I could use that separate 110 VAC coil as a secondary.
It suppose to feed neon lamp only with current less than 0.1 A.

Transformers with twin primaries like the one you linked to on eBay are designed to have both primary windings driven. That's how they work on both 230V and 115V - for 230V in series and for 115V in parallel. That transformer will still work with only one of the two primaries driven, but it will only deliver half the power - i.e. 32.5VA not 65VA as advertised as it'll be running at half the flux. This sounds like it may not be enough for you as you're wanting up to 50VA.